Session ID: extend the regex to match possible hash representations
Improves #306
Relates to #335 & #336
Duplicated by #339
Issues:
- PHP regenerates the session ID if it is not compliant
- the regex checking the session ID does not cover all cases
- different algorithms: md5, sha1, sha256, etc.
- bit representations: 4, 5, 6
Fix:
- `index.php`:
- remove `uniqid()` usage
- call `session_regenerate_id()` if an invalid cookie is detected
- regex: support all possible characters - '[a-zA-Z,-]{2,128}'
- tests: add coverage for all algorithms & bit representations